Surprisingly little is known of the properties of the sympathetic preganglionic neurons (SPN's) of the spinal cord. Thus, electrical membrane properties of these neurons, the characteristics of synaptic transmission between SPN's and their presynaptic input and the nature of transmitter(s) mediating these synaptic potentials were not identified so far. The objective of this proposal is to study the activity of SPN's in vitro. Thin spinal cord slices (0.5-1m.m.) with their corresponding white rami will be obtained from the upper thoracic segments of the neonatal rat spinal cord, and maintained in an organ bath. Intracellular recordings of the SPN's will be carried out by means of glass micro-electrodes. SPN's will be identified by antidromically invaded spike potentials following stimulation of white remus by means of a suction electrode; in addition, the stimulus delay and conduction velocity of the antidromic spike as well as histological verification will be employed to ascertain that the recording neuron is a SPN. First, the passive and active membrane electrical properties of the SPN's will be evaluated. Second, synaptic potentials will be induced presynaptically by means of focal stimulation 100-200 Mum away from the impaled neuron. The time course, input resistance change, reversal potential and ionic mechanisms of evoked potential will be analyzed. Third, the nature of the transmitter(s) responsible for the generation of the synaptic potentials will be defined by using specific pharmacological antagonists to known and putative transmitters, and by attempting to mimic the potentials by application of substances likely to serve as transmitters, particularly those compounds that exhibit excitatory actions on other neurons, such as ACh, glutamate, serotonin and substance P. Forth, effects on SPN's and on evoked SPN transmission of substances that may modulate this transmission, such as catecholamines GABA, and enkephalins as well as action of centrally active antihypertensive drugs such as clonidine and methyldopa, and hypertensive agents such as angiotensin will be examined. The proposal will contribute to our knowledge of the biophysical behavior of SPN's and their role in the central regulation of cardiovascular system as well as the mechanism of action of several centrally acting antihypertensive drugs. In the wider sense, the study will add to our knowledge, at present so deficient, of neurotransmitters and neuro-modulators which are active in the spinal cord.